On Wednesday, North Korean state media KCNA announced leader Kim Jong Un had visited the country's Chemical Material Institute of the Academy of Defense Sciences.

"He instructed the institute to produce more solid-fuel rocket engines and rocket warhead tips by further expanding engine production process," the statement said.

But it was the photos of the inspection released by state media which missile analysts seized upon immediately.

"This is the North Koreans showing us, or at least portraying, that their solid-fuel missile program is improving at a steady rate," David Schmerler, research associate at the Middlebury Institute of International Studies at Monterey's James Martin Center for Nonproliferation Studies, told CNN.

One photo of Kim reveals a poster on the wall clearly mentioning a missile called "Pukguksong-3," a potential successor to the previous two versions of the missile which were both solid-fuel, medium-range projectiles.

North Korea's desire to build solid-fuel missiles is driven by their need for projectiles they can launch quickly and subtly, said Michael Duitsman, also a research associate at the James Martin Center.

"Solid fuel missiles are much faster to deploy ... a solid fuel missile is always fueled so all they have to do is drive it to the place they want to launch it," he said.

"It's much easier to put into action, much harder to catch before it launches because they're a lot less in terms of launch preparations that could be done."

All ballistic missiles owned by the United States and Russia are solid-fuel models, according to Duitsman.

In another, the North Korean leader stands next to a large copper-colored container, which experts said could be a wound-filament reinforced plastic rocket casing.

"It's not a missile test but it's still very disconcerting for people who look at the North Korean ballistic missile program," Duitsman.

"Seeing the casing ... is sooner than I expected."

Both experts said the wound-filament casing seen in one photo would be lighter than previous metal versions, allowing North Korea's missiles to fly further.

When the US Navy first switched to the lighter casing during the 1960s their missiles flew an additional 500 miles, an increase of about 50 per cent, Duitsman said. "They also switched the propellant (though)," he added.

Schlermer said it was unlikely that either the revelation of the new missiles or the filament casing were a mistake by Pyongyang.

"I don't think there's any accident about this, the shot clearly shows Pukguksong-3, this was the North Koreans showing us what we could possibly see soon," he said.

Trump: Kim is 'starting to respect us'

High-profile US leaders have praised Pyongyang for showing "restraint" in pulling back from its previous pledges to launch missiles into the sea around Guam.

One week ago, US President Donald Trump sent a tweet saying Kim had made a "wise decision" not to launch a missile, adding the alternative would have been "both catastrophic and unacceptable."

Speaking at a rally in Arizona Wednesday, Trump claimed Kim was "starting to respect us."

"I respect that fact very much. Respect that fact. And maybe probably not, but maybe, something positive could come about. (The media) won't tell you that. But maybe something positive could come about," he told supporters.

US Secretary of State Rex Tillerson has also presented a more conciliatory face to North Korea in recent days, saying the US was open to dialogue with the rogue state.

"I think it is worth noting, we have had no missile launches or provocative acts on the part of, or provocative actions, on the part of North Korea since the UN Security Council resolution" sanctioning Pyongyang on August 5, Tillerson said Tuesday.

"I am pleased to see that the regime in Pyongyang has demonstrated restraint. We hope this is the signal we have been looking for, that they are ready to restrain provocative acts. And perhaps we are seeing a pathway in the near future to having some dialogue"